Hydrazinium 5-amino tetrazolate is one of the most important explosive materials which is produced by the reaction between hydrazine and 5-amino tetrazole. In the present study, the intermolecular interaction of hydrazine with seven stable tautomers of 5-amino tetrazole is calculated in the gas phase and different solvents including water, ethanol and DMSO. For this purpose, twenty five configurations of hydrazinium 5-amino tetrazolate were obtained by optimizing calculations using the density functional theory (DFT) and perturative method separately, and the value of interaction energy for each configuration has been reported. Using the experimental standard enthalpy formations (?fH?(g)) of hydrazine, 5-amino tetrazole and calculating the interaction energy for each configuration, the ?H?f,gas for each configuration of Hydrazinium 5-amino tetrazolate is obtaind. Finally, considering the population ratios of different configurations and their standard enthalpy formations a single value for ?fH?(g) of 5-amino tetrazolate at two employed levels of theory is obtaind.